Photographic processes for obtaining direct positive images without the use of a reversal processing step or negative film have been well known.
Methods for forming positive images by using conventional direct positive silver halide photographic materials are roughly divided into two types based upon their practical usefulness.
In one type, a silver halide emulsion which has previously been fogged is used. Solarization or the Herschel effect is used to destroy the fogged nucleus (latent image) of the exposed portions so that direct positive images are obtained after development.
In the other type, an unfogged internal latent image type silver halide emulsion is used. The internal latent image type silver halide emulsion which has been exposed to light is subjected to surface development after or while being fogged so that direct positive images are obtained.
The term "internal latent image type silver halide photographic emulsion" as described above means a photographic emulsion of silver halide grain which contains a light-sensitive nucleus mainly in the inside thereof so that a latent image is formed mainly in the inside thereof by being exposed to light.
The latter silver halide emulsion type generally provides a higher sensitivity than the former and is therefore suitable for applications requiring a high sensitivity. The present invention relates to the latter silver halide emulsion type.
In the art, various methods to form direct positive images have been heretofore known. Main examples of such methods include those described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322 (2,497,875), 3,761,266, 3,761,276 and 3,796,577, and British Patent Nos. 1,151,363 and 1,150,553 (1,011,062).
With these known methods, a relatively high sensitivity direct positive type photographic light-sensitive material can be prepared.
The details of the mechanism of formation of direct positive images are described in "The Theory of the Photographic Process" (edited by T. H. James, pp. 182-193, Chapter 7, 4th Edition) and U.S. Pat. No. 3,761,276.
More particularly, the mechanism is believed to be as follows. A so-called internal latent image (positive hole) is produced in the inside of silver halide when the first imagewise exposure to light is effected. Such a positive hole causes a reduction in surface sensitivity. In this manner, fogged nuclei are selectively produced only on the surface of the unexposed silver halide grains. When an ordinary so-called surface development is then effected, a photographic image (direct positive image) is formed.
As means for selectively forming fogged nuclei as described above, there have been known a process which comprises subjecting the entire surface of the light sensitive layer to a second exposure to light, i.e., a so-called "light fogging process" (as described in British Patent No. 1,151,363) and a process which comprises using a nucleating agent, i.e., a so-called "chemical fogging process". The latter process is described in, for example, Research Disclosure, No. 15162, Vol. 151, pp. 72-87 (Nov., 1976).
The formation of direct positive color images are generally accomplished by a process which comprises subjecting an internal latent image type silver halide material to surface color development after or while being fogged, and then subjecting the light-sensitive material to bleach, fixing (blix), and ordinary rinsing and/or stabilization.
In the conventional chemical fogging process, a compound which serves as a nucleating agent only at a high pH of 12 or more is used. Therefore, this fogging process is disadvantageous in that the developing agent is susceptible to deterioration due to aerial oxidation at such a high pH. This will result in a remarkable reduction in development activity. Furthermore, this fogging process allows only a low development speed and thus consumes a long processing time, especially when a developing solution of a low pH value is used. Even when the pH value is 12 or more, the development takes much time.
On the other hand, the light fogging process does not require such a high pH condition and thus can be advantageously applied for practical use. However, this fogging process is not advantageous for all of the various uses required in the photographic field. That is, since the light fogging process is based on the formation of fogged nuclei by photodecomposition of silver halide, different types and properties of silver halide used provide correct exposure illuminances and exposures. Therefore, the light fogging process is disadvantageous in that it is difficult to provide a constant property and requires a complicated and expensive developing apparatus. This fogging process is also disadvantageous in that it consumes a long development time.
Thus, both of the conventional fogging processes fail to provide stable, excellent direct positive images. As means for solving these problems some compounds which serve as nucleating agents have been proposed in Japanese Patent Application (OPI) No. 69613/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and U.S. Pat. Nos. 3,615,615 and 3,850,638. However, these nucleating agents are disadvantageous in that they act on silver halide or undergo decomposition during storage in the light-sensitive material before processing. This results in a reduction in the maximum image density after processing.
A process which comprises speeding up the development of the maximum image density by use of a hydroquinone derivative is described in U.S. Pat. No. 3,227,552. However, even with this process, a sufficiently high development speed cannot be provided, especially when a developing solution of a pH value of 12 or less is ued.
A process which comprises raising the maximum image density by incorporation of a mercapto compound containing a carboxylic acid group or sulfonic acid group is described in Japanese Patent Application (OPI) No. 170843/85. However, the incorporation of such a mercapto compound gives only a small effect.
A process which comprises processing a light-sensitive material with a processing solution (pH 12.0) containing a tetraazaindene compound in the presence of a nucleating agent to lower the minimum image density so that the formation of a re-reversal negative image is prevented is known (Japanese Patent Application (OPI) No. 134848/80). However, this process can provide neither a high maximum image density nor a high development speed.
A light-fogging process which comprises incorporating a triazoline-thione or tetrazoline-thione compound as a fog inhibitor in a light-sensitive material forming direct positive images thereof is described in Japanese Patent Publication No. 12709/70. However, this process, too, can provide neither a high maximum image density nor a high development speed.
Thus, there have been no processes for producing direct positive images having a high maximum image density and a low minimum image density in a short period of time.
In instant color photography (color material dispersion transfer process), an image can be obtained in a short period of time. However, this photography demands a higher development speed.
In general, a high sensitivity direct positive emulsion is more susceptible to generation of a re-reversal negative image at a high intensity exposure condition.